A voltage mirror is a circuit that forces two of the nodes in a circuit to have the same voltage potential. The voltage being mirrored is called the reference voltage (VX), and can be considered as the input of the circuit, and the voltage mirrored is called the mirror voltage (VY), and can be considered as the output of the circuit. In some applications, a high-gain high-speed operational amplifier may be used to implement a voltage mirror, as shown in FIG. 1. The reference voltage VX is connected to the positive input terminal of the operational amplifier, while the output terminal is connected to the negative terminal of the operational amplifier. The feedback mechanism forces the negative terminal to have the same potential as the positive terminal, such that VY=VX, and VY is then the mirror voltage of VX. The performance of the operational amplifier, such as steady state error, transient response, minimum supply voltage, power consumption and dynamic range, determines the accuracy of the voltage mirroring. If the supply voltage is very low, the design of the operational amplifier with a high gain, high bandwidth, wide input common mode range, wide output swing and low power consumption is a very challenging task.
In some applications, a voltage mirror may be implemented by using a matched current source technique, as shown in FIG. 2. If two transistors have the same corresponding gate, drain and source voltages, they will have the same current densities, where the current density of a MOS transistor is defined as the ratio of its drain current to its aspect ratio (W/L ratio). In FIG. 2, transistors M201 and M202 with the same W/L ratio are biased with two matched current sources Ib1 and Ib2, such that Ib1=Ib2. Therefore, M201 and M202 have the same gate to source voltages, and with a common gate configuration, their source voltages VX and VY are forced to be the same. This voltage mirror is very simple and the speed is moderate, but it suffers from systematic offset error introduced by the different drain to source voltages of M201 and M202, and the mirroring accuracy is not high. The two current sources may be replaced by a self-biased structure composed of transistors M303 and M304, as shown in FIG. 3. However, systematic offset exists for M303 and M304, because they have different drain to source voltages, and the mirroring accuracy is not high.
One major application of voltage mirrors is in designing integrated current sensors that are widely used in switching converters for current mode control and over-current protection. Prior approaches include using current sensing resistors and current transformers. Sensing resistors dissipate much power, and current transformers are too bulky and expensive. Integrated current sensors dissipate a very small power and their sizes are small compared to the power transistors, and the production cost can be much reduced.
FIG. 4 shows an example of using a voltage mirror in sensing the current through the power transistor M401. The size of the power transistor M401 to the size of the sensing transistor M402 is N:1, with N>1. The voltage mirror forces the voltages VX and VY to be equal, and the current density of the two transistors are then the same. With M401 N times larger than M402, then I1=NI2. Therefore, the main current of M401 is monitored by a much smaller current of M402.
Current sensing may also be achieved by using current sensing resistors. FIG. 5 shows a current sensing resistor R1 that has a very low value. A traditional method may monitor the voltage across R1, and the current is given by I1=VR1/R1. A voltage mirror may be used instead, and a second resistor R2 that has a value of R2=NR1 is used to sense the current I1. The voltage mirror forces the voltages VX and VY to be equal, and the voltages across R1 and R2 are then equal. With R2 N times larger than R1, then I2=I1/N. Therefore, the main current of I1 is monitored by a much smaller current of I2.
To force the terminal voltages of two transistors or two resistors to be the same, a fast and accurate voltage mirror is required. Therefore it is desirable to construct a high quality voltage mirror with only a few transistors using a very low supply voltage that attains high accuracy, high speed and wide dynamic range.